Consumers purchase meat products after making judgments about the quality and price of the meat. Consumers often use color to judge the ultimate tenderness and taste of the meat product. However, such quality attributes are very difficult to quantify from visual appraisal.
For purposes of this application, the term "meat(s)" or "meat product(s)" are synonymous and refer to the processed cuts of meat offered to the wholesale and retail buying public.
Because most meat products, and fresh meats in particular, are purchased based upon a visual inspection of the product, abnormal coloration has an adverse effect on the salability of the product (Topel et al., 1976; Wachholz et al., 1978). Moreover, pale pork is more susceptible to further discoloration during retail display (Topel et al., 1976; Greer and Murray, 1988).
While the discussion which follows is largely limited to pork, this limitation is for brevity only. The invention described hereinbelow functions with equal success in the treatment of poultry, such as chickens, turkeys, and the like, as well as beef, lamb, and veal.
In order to more objectively judge pork quality, four categories have been developed to describe the visual appearance of fresh pork. These four categories are described in Table 1 as follows (see, for instance, Joo, 1994):
TABLE 1 ______________________________________ Per- cent Category Description Quality pH Drip ______________________________________ RFN Reddish pink, Ideal &lt;6.O &lt;6.0 Firm, Non-exudative RSE Reddish pink, Soft Questionable &lt;6.0 &gt;6.0 and floppy, Exudative PSE Pale pinkish gray, very Poor &lt;6.0 &gt;6.0 Soft and floppy, very Exudative DFD Dark purplish red, very Varied: may be &gt;6.0 &lt;3.0 Firm, Dry (free of juicy, spoil easily surface fluids) ______________________________________
It is known that a rapid fall in pH within pork immediately postmortem, while muscle temperature remains high, results in acidic conditions which give rise to lower quality PSE meat. This effect is most notable in the loins and hams. This rapid fall is believed to be the result of rapid glycolysis of glycogen in muscles which results in denaturation of proteins. The occurrence of PSE is known to be affected by pre-slaughter stress as well as the genetic makeup of the pig. The occurrence of lower quality PSE meat has risen as pigs have been bred to yield leaner carcasses.
From an economic standpoint, PSE meat represents a tremendous loss in value to pork producers because the loins and hams of a slaughtered pig represent approximately one-third of the musculature on the carcass and approximately one-half of the market value of the carcass. Lowered quality of these important cuts of meat greatly reduces the economic return to pork producers and packers.
A 1992 survey of pork quality published by the National Pork Producers Council (NPPC) included data from fourteen slaughtering plants and represented 10,753 gluteus medius muscles (hams). The survey found 16 percent of these hams to be PSE and 10 percent to be DFD pork, proportions the NPPC felt should be of concern to the industry. Importantly, over half of the hams were RSE, possessing normal color, but accompanied by a soft, floppy, and exudative condition. The NPPC recommended, among other things, that color, water-holding capacity, pH, and marbling content be recorded for each carcass and included in every packer report to producers, so that the industry can remain informed of quality variations and can take appropriate steps to improve breeding stock. The NPPC also recommended the adoption of price differentials for differences in quality, similar to those which exist for leanness. These results and recommendations stress a long-felt need to better understand the problem of PSE-RSE so that it can be controlled, predicted, and prevented.
As alluded to above, the economic losses associated with the occurrence of PSE and RSE pork are extensive. Direct costs include reduced yields in the carcass, as well as reduced yields in wholesale and retail cuts used for processing and cooking. Indirect costs include an increase in product variability and a reduction in consumer appeal due to poor color, drip loss in retail display (Kauffman et al., 1978; Smith and Lesser, 1982), variation in organoleptic qualities in cooked hams (Honkavaara, 1988), and reduced juiciness after cooking (Bennet et al. 1973; Jeremiah, 1984).
In 1982, the reduced financial return of a PSE carcass was calculated to range from zero to $16 in the U.K. (Smith and Lesser, 1982). The benefits of lowering the incidence of PSE pork by only 1% were estimated to be worth about $5 million per annum to the Australian pig industry, with 85% of the returns accruing to producers (Voon, 1991). The Pork Chain Quality Audit (Meeker and Sonka, 1994) suggests that pork quality problems currently are costing the industry in excess of $50 million per annum. A similar study by Kaufmann et al. (1996, unpublished), estimates that pork quality problems in the U.S. result in lost revenues of approximately $75 to $140 million annually (1996 dollars).
The water-holding capacity (WHC) of fresh pork and the binding of added water during storage and further processing are also of significance to the pork industry. Water retention is essential for pork palatability in terms of juiciness and possibly tenderness. Furthermore, loss of fluids (often in excess of 7%) results in a reduction in the weight of marketable pork. The occurrence of a pale color and high exudation or a dark color and minimal exudation has led to the assumption that color and WHC are related. However, van Laack et al. (1994) and Warriss and Brown (1987) have demonstrated that color and WHC are not necessarily related, especially within the reddish-pink range (RFN, RSE).
Fresh pork at slaughter contains about 75% water, and about 85% of this muscle tissue water is located intracellularly, primarily in the spaces between the thick and thin filaments. The remaining 15% of the water in fresh pork is located in the extracellular spaces (Hamm, 1975). Changes in WHC result from changes of the filament spacing associated with changes in the charge and structure of the myofibrillar proteins, especially myosin. Swelling or shrinkage of the muscle fiber results in changes in the filament spacing and causes water movement between the intracellular and extracellular spaces (Offer and Knight, 1988). During the conversion of muscle to pork and subsequent aging of pork, the interfilament spacing can be changed by the rate and extent of pH fall, sarcomere length, ionic strength, osmotic pressure and onset of rigor mortis (Hamm, 1994; Offer and Knight, 1988; Wismer-Pederson, 1987).
The occurrence of PSE is known to be affected by antemortem factors such as pre-slaughter stress and the genetic makeup of the individual animal. The PSE condition is more likely to arise in white muscles, i.e. those muscles which have predominantly white fibers. These fibers have the capacity to undergo rapid postmortem glycolysis (pH decline) especially when there is antemortem stress (Warner et al., 1993). As noted above, the pork muscles that are subject to the PSE condition comprise approximately one-third of the total muscle mass.
Recent studies by Ahn et al. (1992), Boles et al. (1993; 1994) and Grzes et al. (1994) indicate that minute quantities of sodium bicarbonate (baking soda) have an elevating and stabilizing effect on the ultimate pH of muscle. However, it must be also noted that sodium bicarbonate was not effective when administered to live pigs prior to slaughter (Boles et al., 1994).